For a variety of economic, political, and environmental reasons there has been a drive for higher fuel efficiency, lower emissions, and more sustainable fuel sources. This drive has resulted in changes to fuel composition and chemistry of middle distillate fuels, new high precision engine designs, and the use of biofuels and blends thereof. These trends are discussed, for example, in “Filtration Solutions for High Pressure Common Rail Fuel Systems,” Barry Verdegan, Abby True-Dahl, William Haberkamp, Norm Blizard, David Genter, and Eric Quillen, American Filtration & Separation Society Annual Conference, May 19-22 (2008), Valley Forge, Pa. As a consequence, new fuels often have higher water content, lower interfacial tension against water, and now readily form stable emulsions of very fine water droplets on shearing in transfer via pumping. In addition, newer precision engine designs are highly sensitive and easily damaged. The escalated water content in fuel when present in the form of free water droplets can reduce lubricity, thus causing damage. Furthermore, suspended particulates of a size from 4-20 microns and smaller can provide a significant source of abrasive wear. These factors generate a need for higher fuel purity requiring durable, long-lasting fuel filters, water separators, and the porous filter media composites that comprise them. These filters find use in a variety of applications where middle distillate fuels are used in combustion engines including, but not limited to: automotive, trucking, marine, and aerospace markets.
Filter media known to be applied for these applications include a wide variety of porous and composite materials. The porous composite media most commonly employed in practice are comprised of fibrous polymer non-wovens (non-wovens” as used herein are also referred to alternatively but equivalently as “nonwovens”, cellulose or paper nonwovens including those containing microfiber glass, and textiles. Many of the aforementioned media also include hydrophobic coatings. Examples also exist of fibrous and non-fibrous microfilter membranes including fully and partially fluorinated polymers and expanded polytetrafluoroethylene (ePTFE).
However, these porous and composite materials fail to meet increased needs for durable, long-life, water and particulate separation. Specifically, there is a need for a porous composite capable of removing water and fine particulate to a purity level sufficient to protect new engines in the new fuel compositions and chemistries which include interfacial tension-lowering additives and surfactants. U.S. Pat. Nos. 5,904,845, 7,938,963, US20090178970 provide examples of attempts which include combinations of microfibrous composite non-wovens and textiles. These attempts fail to provide sufficient emulsified water removal in the presence of additives and surfactants in new fuels. In addition these attempts often include hydrophobic treatments that lack durability and are defeated over time. In contrast, ePTFE microfilters such as described in US patent application 2008/0105629 A1, can provide sufficient durable emulsified water removal, but are found to rapidly clog in the presence of particulate, thus reducing or eliminating liquid passage through the filter rendering the filter inoperable. In contrast, the ePTFE described by U.S. Pat. Nos. 6,764,598, and 7,285,209 is not always effective in preventing clogging and involves a complicated apparatus requiring recirculation to provide sweeping flow across the composite surface to delay clogging. Thus, known proposed solutions which employ ePTFE have issues related to clogging and are of limited practical use.
Significantly, much of the existing art regarding filtering water droplets from fuel involves the use of a “coalescer.” A coalescer operates to remove water from fuel by allowing fine water droplets to pass through the material of the coalescer, but to encourage those fine droplets to merge, or coalesce, with one another. The water thus forms coarse particles or droplets which are then heavy enough to fall out of the fuel, for example by the force of gravity. This approach, which allows the water to flow through the material and encourages fine-to-coarse particle size progression in order to separate water from fuel actually teaches away from the present invention. In accordance with teachings of the present invention, both fine and coarse water droplets are rejected at the surface of a fine separating layer, rather than being allowed to pass through it.